In radiography the interior of objects is reproduced by means of penetrating radiation, which is high energy radiation also known as ionizing radiation belonging to the class of X-rays, γ-rays and high-energy elementary particle rays, e.g. β-rays, electron beam or neutron radiation.
For the conversion of penetrating radiation into visible light and/or ultraviolet radiation “luminescent” substances, called “phosphors”, are used. Cathodoluminescent phosphors employed e.g. in CRT screens exhibit two related luminescent characteristics: fluorescence and phosphorescence. Fluorescence is the luminescent build-up or emission of light released from the phosphor during the time of electron beam excitation. Phosphorescence is the emission of light from the phosphor occurring after the cessation of electron beam excitation. The duration of phosphorescence, or rate of decay of afterglow, is denoted as persistence, usually expressed as a measurement of time required for the phosphorescence in order to reduce or decay to a ten percent level of steady state fluorescent brightness.
In known X-ray image intensifiers for example as disclosed in U.S. Pat. No. 3,838,273, the input screen comprises a substrate such as glass or aluminum on which is deposited an X-ray sensitive radiation conversion layer, commonly referred to as a fluorescence layer or scintillator.
In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays. In practice the light emitted imagewise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
As described e.g. in U.S. Pat. No. 3,859,527 an X-ray recording system has meanwhile been developed wherein photostimulable storage phosphors are used, said phosphors having, in addition to their immediate light emission (prompt emission) on X-ray irradiation, the property to store temporarily a large part of the X-ray energy. Said energy is set free by photostimulation in the form of fluorescent light different in wavelength from the light used in the photostimulation step. In said X-ray recording system the light emitted on photostimulation is detected photoelectronically and transformed into sequential electrical signals. A storage screen or panel coated with such phosphors is exposed to an incident pattern-wise modulated X-ray beam and as a result thereof energy is temporarily stored in the coated storage phospors, corresponding with the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are processed to produce a visible image. Stimulation light can be transformed into an electric signal by making use of a photoelectric conversion element such as e.g. a photo-multiplier. It is clear that the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called “digital radiography” or “computed radiography” (CR).
Recently, in the hospitals the tendency is increasing to obtain X-ray images on computer monitor immediately after X-ray exposure of the patient. By storing and transmitting that digitized information, efficiency and speed of diagnosis is enhanced. Accordingly “direct radiography” providing directly digital diagnostic X-ray images, after exposure of an adapted detector panel in a radiographic apparatus, becomes preferred instead of the conventional screen/film system. The X-ray quanta are transformed into electric signals by making use of a solid-state flat detector as “image pick-up” element. Such a flat detector is commonly called a “flat panel detector” and is two-dimensionally arranged. Making use therein of a photoconductive material as a detecting means, such as a-Se, in which the negative electrical charge of an electron and the positive electrical charge of a hole are generated by the X-ray energy, said X-ray energy is directly converted into those separated electrical charges. The electrical charge thus obtained is read out as an electric signal by the read-out element, two-dimensionally arranged in a fine area unit.
Further on an indirect type flat panel detector is known, in which the X-ray energy is converted into light by a scintillator, and in which the converted light is converted into the electric charge by the photoelectric conversion element such as a-Si two-dimensionally arranged in a fine area unit. The electrical charge is read out again as an electric signal by the photoelectric conversion read-out element, two-dimensionally arranged in a fine area unit.
Moreover a direct radiography detector is known in which the X-ray energy is converted into light by a scintillator, and wherein the converted light is projected on one or more CCD or CMOS sensors which are arranged matrix-wise in the same plane, through a converging body such as a lens or optical fiber. In the inside of the CCD or CMOS sensor, via photoelectric conversion, and charge-voltage conversion, an electric signal is obtained in every pixel. This type of detector is also defined, therefore, as a solid state plane detector.
The image quality that is produced by any radiographic system using phosphor screen or panel, and more particularly, within the scope of the present invention, in a digital radiographic system, largely depends upon the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be. This means that the lower the ratio of binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen or panel, will be. Optimum sharpness can thus be obtained when screens without any binder are used. Such screens can be produced, e.g., by physical vapor deposition, which may be thermal vapor deposition, sputtering, electron beam deposition or other of phosphor material on a substrate. Such screens can also be produced by chemical vapor deposition. However, this production method can not be used to produce high quality screens with every arbitrary phosphor available. The mentioned production method leads to the best results when phosphor crystals with high crystal symmetry and simple chemical composition are used. So in a preferred embodiment use of alkali metal halide phosphors in storage screens or panels is well known in the art of storage phosphor radiology and the high crystal symmetry of these phosphors makes it possible to provide structured, as well as binderless screens.
It has been disclosed that when binderless screens with an alkali halide phosphor are produced it is beneficial to have the phosphor crystals deposited as some kind of piles or pillar-shaped blocks, needles, tiles, etc., in order to increase the image quality that can be obtained when using such a screen. In U.S. Pat. No. 4,769,549 it is disclosed that the image quality of a binderless phosphor screen can be improved when the phosphor layer has a block structure, shaped in fine pillars. In U.S. Pat. No. 5,055,681 a storage phosphor screen comprising an alkali halide phosphor in a pile-like structure is disclosed. In EP-A 1 113 458 a phosphor panel provided with a selected vapor deposited CsBr:Eu phosphor layer is disclosed, wherein the binderless phosphor is present as fine needles in favor of an optimized image quality.
It is clear that, from a point of view of homogeneity of layer thickness and chemical composition of the scintillator, in favor of a constant speed, image quality and diagnostic reliability, it is of utmost importance to provide said constant composition and layer thickness over the whole two-dimensional panel surface in the production of the storage phosphor plates. A homogeneous coating profile should thus be strived for.
Manufacturing procedures making use of deposition techniques as in U.S. Pat. No. 4,449,478 wherein an arrangement for coating substrates in an apparatus for vacuum deposition comprises a rotatable substrate holding structure in a form of a plate, provide coated panels limited in number and in coated surface. Moreover, when square or rectangular panels are desired, quite a lot of expensive residue is created while not being deposited onto the plate, rotating with a circular symmetry above the vapor source. In addition, when depositing material from a fixed source onto such a rotating substrate, it is difficult to obtain a layer with a constant thickness. Circles on the substrate, having the rotation axis as centre will have identical deposition history. This creates a centro-symmetric thickness profile, which means that the thickness of the deposited layer is constant along neither of the sides of a rectangular substrate, unless special precautions are taken, e.g. use of a mask which will however lead to material losses.
Manufacturing procedures make use of e.g. deposition techniques as in U.S. application 2003/0024479, with an arrangement for coating rigid substrates, batch wise (plate per plate), in an apparatus for vacuum deposition. Such an apparatus comprises a substrate holder conveyed from a loading chamber to an unloading chamber by a conveying mechanism, and provides coated panels limited in process yield. Another batch process has been described in U.S. Pat. No. 6,402,905, where commonly a vapor deposition coating process is applied by which vapor is deposited onto a substrate that rotates around one axis that is perpendicular to the deposition area, as commonly applied in the state of the art, and wherein a system and method for controlling and compensating unevenness of the deposition thickness distribution on a substrate has been described.
In such a batch process a coating failure will generally lead to the complete loss of a panel. Moreover there will always be a high loss of raw materials in such batch processes. Lack of deformability of the substrate also limits the format of the coating of a phosphor or scintillator layer on a substrate to the largest cross section of the vacuum deposition chamber.